1. Field of the Invention
This invention relates to a failure determination system and method for a plurality of temperature sensors provided in an internal combustion engine, and an engine control unit.
2. Description of the Related Art
Conventionally, this kind of failure determination system for temperature sensors has been proposed in Japanese Utility Model Publication (Kokoku) No. H05-24033. The failure determination system is provided for determining failures of an intake air temperature sensor for detecting the intake air temperature of the engine, and an engine coolant temperature sensor for detecting the coolant temperature of the engine. More specifically, first, detection values of the intake air temperature and the engine coolant temperature detected by the intake air temperature sensor and the engine coolant temperature sensor, respectively, are sampled at predetermined time intervals for a predetermined time period after stoppage of the engine. Then, final convergence values of the detection values of temperatures detected by the respective temperature sensors are calculated (estimated) using the sampled detection values, by predetermined approximate expressions. When the difference between the convergence values is not larger than a predetermined value, it is determined that both of the temperature sensors are normal, whereas when the difference between the convergence values is larger than the predetermined value, it is determined that at least one of the temperature sensors is faulty. The above determination method utilizes the fact that when the temperature sensors are both normal, the detection values of temperatures detected by the respective temperature sensors converge to ambient temperature, such as outside air temperature, and finally become approximately equal to each other.
As described above, in the failure determination system, the final convergence values of the detection values are estimated based on the detection values of temperatures detected by the respective temperature sensors. Therefore, the accuracy of the estimated convergence values is not necessarily high, which sometimes prevents accurate failure determination of the temperature sensors. Of course, it is possible to enhance the accuracy of the convergence values by sampling the detection values detected by the temperature sensors for a longer time period. In this case, however, it takes a long time to determine failures of the temperature sensors.
Further, some temperature sensors are used in actual operating temperature ranges largely different from ambient temperature. In such a case, even if convergence values obtained from the respective temperature sensors are compared with each other, it is impossible to carry out proper failure determination of the temperature sensors in actual operating temperature ranges thereof. For example, in general, exhaust gas temperature sensors for detecting the temperature of exhaust gases are not configured to be capable of ensuring sufficiently high detection accuracy in a normal temperature range, including outside air temperature, since they are used in a very high temperature range of which values are 300 to 400° C. even immediately after stoppage of the engine. Therefore, it is impossible to properly determine failures of the two temperature sensors in the operating temperature range by comparison between the convergence values which are obtained from the respective temperature sensors but are largely different from the operating temperature range.